Sensitizers have been associated with photoacid precursors for the initiation of cationic photopolymerization. Generally this association allows for the photoacid precursor to be activated at a wavelength of radiant energy for which the photoacid precursor has low absorbance. The use of such sensitization of photoacid precursors for initiation of cationic polymerization, in particular polymerization of epoxy functional resins has been described in U.S. Pat. No. 5,055,439 to Allen et al., U.S. Pat. No. 4,069,054 to Smith and, U.S. Pat. No. 4,250,053 to Smith. Recommended concentrations of the sensitizer are from 0.01 to 1 part by weight of photoacid precursor used for the polymerization. The concentrations of sensitizer taught, in particular for 1,6-diphenyl-1,3,5-hexatriene and 1,8-diphenyl-1,3,5,7-octatetraene, are excessive when sensitizing a triarylsulfonium photoacid precursor salt for use with an argon ion laser operating with two major wavelengths in the UV.
U.S. Pat. No. 4,624,912 to Zweifel et al. discloses numerous sensitizers, in particular 1,12-benzperylene, for use with photoacid precursors. Recommended concentrations of up to 10% by weight. Such concentrations of a sensitizer, such as 1,12-benzperylene, are excessive in solutions using triarylsulfonium salts, for applications using an argon ion laser in the UV, such as solid imaging, in which a depth of penetration greater than 2.5 mils is preferred.
In solid imaging or sterolithography, the thickness of a coating layer is, in some instances, only defined by the depth of penetration of the light below the surface of the liquid. For example in solid imaging using a cationically cured epoxy in a liquid mixture, the depth of the liquid may be much greater than the depth that is to be photohardened. This is particularly true in imaging of a cantilevered structure.
In cantilevered sections, the absorbance of the actinic radiation, to a given depth below the surface of the photopolymer, dictates the depth of layer polymerization. This depth of polymerization will depend upon the exposure and the ability of the photopolymer to reach a degree of insolubilization, due to polymerization. For a given exposure, at some point below the surface of the photopolymer, the absorbance of the actinic radiation by the initiator, is insufficient to induce polymerization such that the polymer formed is integral with the polymer at a lesser depth. At this point, the non-integral polymer may be dissolved in the deeper photopolymer medium, or the polymerization may be terminated, by oxygen, for example, in the case of free-radical polymerizations; or by the formation of a salt, for example, in cationic polymerizations. At higher or lower exposures, the depth of polymerization will be greater or lesser, respectively.
In a solid imaging system where a multi-wavelength laser or other light source is used, in order to achieve some degree of control in determining the depth of penetration and therefore, the depth of photohardening; the absorbances of the photoacid precursor and the sensitizer need be balanced. Otherwise the depth of photohardening from the penetration and activation of the light at one of the wavelengths of the multi-wavelength laser may extend far below the depth of penetration and activation of the other wavelengths.
The absorbance depth of the initiator(s) or sensitizers for the initiators, may be vastly different, or the actinic efficiency may occur at significantly different depths. For example, at a given exposure one wavelength of actinic radiation may cause a very small depth of polymerization while another wavelength may cause a very large depth of polymerization. In this case, the energy of the former wavelength is essentially wasted since the latter wavelength energy controls the depth of polymerization as a function of the exposure at that wavelength.
An imbalance in the penetration of the light may result in different depths of photohardening for each corresponding wavelength and can directly cause distortion and loss of tolerance in the imaged object.
The present invention provides a balanced concentration of photoacid precursor and sensitizer optimized for use with a multi-wavelength laser in the solid imaging process. In particular, the argon ion laser. The balanced absorbances (concentration times the molar extinction coefficient) of photoacid precursor and sensitizer provide a control of the depth of penetration of the light and thus the depth of photohardening at each major wavelength emitted by the laser so that a layer having high tolerance and low distortion can be efficiently imaged.
An objective of the present invention is to provide a combination of photoacid precursor and a sensitizer for use with the argon ion laser operating in the UV with at least two wavelengths which provide activation energy for the formation of the photoacid. The concentration and the absorption per mil of the photoacid precursor and the sensitizer are balanced such that the activity of the sensitized photoacid is approximately equal for at least two wavelengths.